The present invention relates to a tire pressure monitoring device and method for monitoring, for example, pneumatic pressure in automobile tires for low and high preset values of pressure for the tires. The present invention has been found to be particularly useful in the automobile pneumatic tire pressure monitoring art, especially as a system for monitoring the pneumatic pressure in any size tire for low pressure, and hence will be discussed with particular reference thereto. However, the present invention is applicable to many other types of tires as well, not only automobile tires, as long as pneumatic tires are used to support the vehicle (one or more tires), and the term "automobile" is used herein to include vehicles of all sorts such as airplanes, tractors, and trucks.
In the monitoring of any type of pneumatic tire for pressure over or under a preset limit, pressure monitoring means is usually attached in some way to the rotating pneumatic tire to permit fluid motion from the air or other inflating gas in the pneumatic tire to the pressure monitoring means. Therefore, additional means must be included to transfer the information obtained by the pressure monitor means to alarm circuit means which would be used to alter the vehicle operator of the high or low pressure. This involves the transference of information by transmission means from the pressure monitor means located on the rotating part of the automibile, the tire, to the non-rotating portion of the automobile where the alarm circuit means is located. This transmission means must be capable of transmitting the signals between the moveable member and the relatively stationary portion of the vehicle in a manner such that environmental factors will not affect the signals transmitted, the device is reliable in operation for a long period of time and consistent in its indication, and vehicle operation is not impeded.
Several types of tire pressure monitoring devices and methods have been known and used before, and typical examples thereof in the pneumatic tire pressure monitoring art are shown in U.S. Pat. No. 3,370,459, issued Feb. 27, 1968 to A. Cescati, U.S. Pat. No. 3,523,450, issued Aug. 11, 1970 to A. Openshaw et al, U.S. Pat. No. 3,421,144, issued Jan. 7, 1969 to E. G. Bustos, U.S. Pat. No. 2,846,664, issued Aug. 5, 1968 to H. J. Amundsen, Jr., et al, U.S. Pat. No. 1,851,978, issued Apr. 5, 1973 to W. H. Dinger, U.S. Pat. No. 3,662,335, issued May 9, 1972 to K. Fritze, U.S. Pat. No. 3,691,524, issued Sept. 12, 1972 to H. Frost, et al, U.S. Pat. No. 3,713,092 issued Jan. 23, 1973 to B. Ivenbaum, U.S. Pat. No. 2,070,743 issued Feb. 16, 1937 to J. W. McDonnell, U.S. Pat. No. 3,588,814, issued June 28, 1971 to T. Furlong, and U.S. Pat. No. 3,665,387 issued May 23, 1972 to R. S. Enabnit.
The Cescati, Amundsen, Openshaw, Dinger, and Bustos tire pressure monitor devices utilize electric brush means to transmit signals from pressure monitor means, such as pressure switches, position plunges, and differential switches to alarm circuit means. However, these techniques are inherently unreliable because of mechanical aspects involved in the coupling such as brush wear and contamination in slip ring type systems.
The Fritze and Ivenbaum devices require, at least in part, active devices requiring continuous power in the rotating member, the tire, of the vehicle transmitting either a radio signal or an oscillator signal to another receiver or coupling device located on the vehicle frame. However, this type of prior art is difficult to implement inside the tire because the power supply inside the tire or at the stem of the tire needs to be installed securely and the wearing out of the power supply, either mechanically if it is self generated by the rotation of the wheel or by decay of a self-contained power supply such as a battery, over a period of time. This, of course, presents inherent unreliabilities with the devices, as well as cost in implementation and maintenance, and the operator will lack confidence in the integrity of the system.
The Frost and Furlong pressure monitoring devices use a type of magnetic coupling means to transmit the signal from the pressure monitoring means to the alarm circuit means. The Frost and Furlong devices use counting means in their magnetic coupling to couple the information from the pressure monitoring means to the alarm circuit means, counting the number of revolutions per wheel to determine tire speeds which can be related to the pressure in the tire. The Frost device requires complex electronic circuitry to perform the count in order to avoid false signals such as would develop from turning. This electronic circuitry, which is subject to shock from the vehicle when it bounces and other hostile environmental factors, yields failure rates that will be much higher than a very simple coupling device. The Furlong device uses a reed relay to count revolutions, which is, therefore, subject to failure since the relay will close every revolution, and compares speed between tires. This prevents distinguishing low tire pressure if all tires were low, and the device also has no circuitry means for distinguishing one tire slowing down in comparison to the others because of turns, etc. It is also therefore inherently unreliable because of its mechanical coupling for the transmission means for the reed relay and other system defects that would require additional circuitry which will also make it unreliable from a complexity standpoint. Readings therefore would result in anomolous indication and a lack of confidence in the integrity of the system by the operator.
The Snabnit, McDonnell, and Fritze tire pressure monitoring devices also use magnetic coupling means between the pressure monitoring means and the alarm circuit means. They either use a pressure switch or pressure diaphram changing the core of a coil from air to a ferrous metal to indicate changes in the pneumatic pressure. The pressure monitor means magnetically couples to the alarm detection circuit means through an oscillator portion of the circuit. However, the circuit is complex so that environmental factors affect the transmitted signal, and, therefore, the circuit lacks reliability because of the complexity of the circuit resulting in anomolous indication and a lack of confidence in the integrity of the system, or the circuit is also provided with self-diagnosing circuitry which makes the circuit even more complex and subject to additional problems of integrity of the system, making them expensive to manufacture and to maintain.
It is, therefore, desired in the pneumatic pressure monitoring art to provide coupling means between the rotating tire and the relatively stationary portion of the vehicle which requires no power source or active components on the movable tire, which is very reliable, which is immune to the effect of the environment, and which is sufficiently sure in reporting to provide reliable and consistent alarm means for pressure changes in the tire.